Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. Examples of such systems can be found in Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, 4,526,442, 4,697,892, and 4,801,196; Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681; and Toide, U.S. Pat. No. 5,148,320.
Color images for projection televisions are normally obtained by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. The phosphors used in commercially available CRTs do not emit light at a single wavelength. In particular, green phosphors have significant sidebands in blue and red. Similar polychromaticity exists for red and blue phosphors, but to a lesser extent.
For many consumer applications, lens systems uncorrected for color can be used, notwithstanding the color spread of the CRTs. For more demanding applications, however, such as high definition television, data displays, or systems which operate at a high magnification, color correction is needed to avoid visible color fringing and/or a loss of image contrast. Examples of projection lens systems which provide at least some color correction include Betensky, U.S. Pat. No. 4,815,831; Kaneko et al., U.S. Pat. No. 5,404,246; Kreitzer, U.S. Pat. Nos. 4,900,139, 5,309,283, and 5,455,713; Moskovich, U.S. Pat. No. 4,963,007; Toide, U.S. Pat. No. 5,130,850; and Yoshioka, U.S. Pat. No. 5,237,456.
A long standing problem in the field of projection lenses for projection televisions has been the provision of a high performance lens system which is optically coupled to a CRT and yet can be focused over a wide range of conjugates without substantial deterioration in the lens' performance. As is known in the art, optical coupling occurs when the interface between the lens system and the CRT is through a coupling fluid (liquid), as opposed to air. As a result of such liquid coupling, the entire lens system cannot be moved relative to the CRT for focusing at different conjugates, as can be done when the coupling is through air. Rather, a portion of the lens system must remain fixed to the CRT to retain the coupling fluid in place. Accordingly, focusing of the lens system involves dividing the system apart. Prior to the present invention, such dividing to focus has resulted in a substantial decrease in the performance of the system, i.e., a substantial increase in the system's aberrations, especially when the focusing has been over a wide range of conjugates.
The ability of a projection lens system to focus over a wide range of conjugates is particularly important for front projection systems used with large viewing screens in auditoriums, lecture halls, and the like. As is commonly known, such viewing screens come in a variety of sizes. What manufacturers and customers both desire is a projection television which can accommodate such size variation without the need to substitute lens elements and without sacrificing performance.
In optical terms, to meet this need, the projection lens used in the projection television needs to be focusable. The present invention addresses this need by providing a fully color corrected, optically coupled, front projection, lens system which can be focused over a wide range of screen sizes (screen diagonals) and can still maintain high performance as evidenced by a MTF which remains substantially uniform over said range of screen diagonals.